Tidal flexing of Io

. Due to an orbital resonance with nearby Europa, Jupiterís satellite Io has a noncircular orbit. The forced eccentricity makes Io travel at different velocities along its orbit and the side facing Jupiter nods back and forth slightly, as seen from the planet. Although only half a degree in extent, this movement causes varying tidal forces inside the satellite, flexing it in and out like squeezing an exercise ball with your hand. This, in turn, generates internal friction and heat, leading to the active volcanoes seen on Io with instruments aboard the Voyager 1 and 2 and Galileo spacecraft. In this drawing, Ioís size is exaggerated when compared with Jupiter.